1. Field of the Invention
The present invention is directed towards method and apparatus for avoiding image flicker in an optical projection display.
2. State of the Art
Optical projection displays generate images by modulating the polarization of certain regions of the light, while leaving the polarization of other regions unchanged. Such displays typically include one or more reflective or transmissive light valves.
FIG. 1 presents a typical reflective light valve for a liquid-crystal-on-silicon (“LCOS”) projection microdisplay. This light valve 100 includes a reflective spatial light modulator (“SLM”) 105 and an output analyzer 110. The SLM 105 receives linearly polarized light. As shown in this FIG. 1, the SLM 105 includes a layer of liquid crystal material 140 that is positioned between two electrodes 115 and 125.
Electrode 115 is a transparent electrode that is deposited on the surface of a transparent cover 120, while the electrode 125 is a reflective electrode that is located on the surface of a semiconductor substrate 130. The transparent electrode 115 is not segmented, while the reflective electrode is segmented (i.e., pixilated) into an array of pixel electrodes 135 that define the pixels of the SLM. (A substantially reduced number of pixel electrodes are shown in FIG. 1 to simplify the drawing.)
Each pixel electrode reflects the portion of the incident polarized light that falls on the pixel electrode. Each pixel electrode can also change the polarization of the light falling on it based on the electrical signals that it receives. Specifically, the potential difference between each pixel electrode and the transparent electrode establishes an electric field across the portion of the liquid crystal material that is between the pixel and transparent electrodes. This electric field in conjunction with the structure and orientation of the SLM's liquid crystal material, determine how the pixel electrode rotates the polarization of light falling on it.
The output analyzer 110 receives the light reflected by the SLM 105. This output analyzer is a polarization-selective device.(such as a polarizing filter or polarizing beam splitter) that allows a certain polarization state of the light to pass, while discarding the remaining polarization states. Hence, the output analyzer is placed at the output of the SLM to obtain the SLM's pattern of modulation, and thereby generate an image.
In addition to using light valves, some optical projection displays also include polarization compensators. A polarization compensator is an active polarizing switch that receives electrical signals that control how the polarizing switch changes the polarization of the light.
FIG. 2 presents a partial view of a projection display 200 that includes a reflective SLM 205 and a polarization compensator 210. The polarization compensator 210 allows the display to drive the SLM 205 in two modes, an inversion mode and a non-inversion mode.
The projection display 200 operates the SLM 205 in these two modes in order to avoid the “sticking” of the SLM's pixels. Sticking is a commonly recognized problem of liquid crystal displays. Sticking occurs when a pixel is left energized for an extended period, causing impurities in the liquid crystal comprising the pixel to migrate. The migration of impurities, in turn, introduces a polarization vector in the liquid crystal at the location of the pixel. This polarization vector can then offset any electric field that is applied across the pixel, and thereby prevent the pixel from switching. Such a pixel is referred to as a “stuck” pixel.
One way of avoiding sticking is to alternate the bias across each pixel during an inversion period. During the inversion period, however, the image is inverted. Therefore, the polarization compensator 210 is used to recover a positive image during the inversion period. More particularly, the driving of the polarization compensator 210 and the SLM 205 is synchronized so that these two devices provide a positive image to the viewer during inversion and non-inversion periods.
Operating a projection display in inversion and non-inversion modes introduces flicker in the displayed image. Specifically, one or more components of projection display operate non-ideally during the inversion period, the non-inversion period, or both periods. Such non-ideal operations cause these components to operate asymmetrically during the inversion and non-inversion periods. For instance, in FIG. 2, the polarization compensator 210 might rotate the polarization of the light by +24 during an inversion period and by −21 during a non-inversion period, instead of ideally rotating the polarization by ±22.5 during these two periods.
Such non-ideal, asymmetric operations cause the projection display to output light asymmetrically during the inversion and non-inversion periods. In other words, the asymmetric operations of the display's components introduce undesired, uniform intensity variations between frames projected during the inversion periods and frames projected during the non-inversion periods.
The viewer perceives the undesired, uniform intensity variations as image flicker. The degree of image flicker also varies with the temperature and voltage of the display, because the asymmetrical behavior of the components (such as the asymmetrical behavior of the polarization compensator) is a function of the temperature and voltage.
One prior art solution for minimizing flicker is to closely match the characteristics of the SLM's and compensators to achieve symmetric light throughput during both inversion and non-inversion periods. Such a solution requires precise matching of the optical properties of the SLM's and the compensators. Hence, this solution is difficult to achieve. It is also expensive, and it is not always effective, especially as time passes.
Another solution is to closely control the temperature variance of the compensator by adding circuitry to heat the compensator. This solution, however, involves the additional cost of the temperature control systems. It also complicates the structure of the polarization compensator.
Therefore, there is a need in the art for a method and apparatus that can avoid image flicker due to the asymmetrical operation of one or more components of a projection display during inversion and non-inversion periods.